Anti-p-azobenzenearsonate antibodies (IgG1 and IgG2), raised in young (2 to 4 months) and 2-3 years old guinea pigs of inbred strain 13, were compared with respect to their binding affinities for the haptens, their amino acid compositions, cyanogen bromide peptide profiles obtained from the molecular seiving columns, amino acid sequences, affinity labeling patterns, and heterogeneity or changes in idiotypes as judged by isoelectric-focusing gels. Thus far, minor differences in the two age groups have been noted. The significance in these differences is being determined.